The present invention relates to a material for use in light modulation devices either by variable reflection of light or by variable transmission of light. Also within the scope of this invention are various electro-optical devices using the material such as windows, screens, automotive windshields and sunroofs, obturators, variable reflection rear view mirrors, displays, etc.
In particular, the invention relates to a material where the light modulation effect is obtained through the reversible electrodeposition of a metallic layer onto a transparent electrode surface from the metallic ions contained in the material,
Light modulation devices are known where two glass sheets coated with an electrically conducting transparent layer made of tin oxide or indium tin mixed oxide are separated by a thin layer of an electrolytic material, thereby constituting an electrolytic cell.
When a voltage is applied across the electrodes of such a cell, an absorbing and/or reflecting metallic layer appears onto the negatively charged (cathode) electrode due to the electrochemical reduction of the metallic ions that are present in the material. Simultaneously, an oxidation electrochemical reaction occurs at the positively charged electrode (anode). These two reactions have to be either electrochemically or chemically reversible in order to allow the cell to return to its maximum transmission state, either by applying a reversed voltage, or by shorting the electrodes, or at least by spontaneous bleaching in open circuit. It might be desirable for applications such as windows or rear view mirrors to keep the cell in a defined light reflection and/or absorption state during a period of time up to several hours before restoring the initial maximum transmission state.
Examples of such devices for use in small area displays where the optically active layer is silver are described in French Pat. Nos. 2,260,167, 2,352,361, 2,468,179 and 2,618,571 and U.S. Pat. Nos. 3,245,313 and 3,626,410. U.S. Pat. No. 3,153,113 relates to large area variable transmission devices. These devices of the prior art use water-based electrolytic materials. In this case, the addition of a strong mineral acid is needed in order to avoid hydrolysis of the metallic salts in solution. These materials generally exhibit low pH and have a tendency to favor hydrogen evolution concurrently to the metal electrodeposition, and/or to induce the degradation of the electronically conducting transparent materials. Organic electrolytic materials have also been proposed, such as those based on methanol (See, e.g., French Pat. No. 2,352,361) or a methanol/acetonitrile mixture (French Pat. No. 2,468,179), which exhibit a poor thermal stability due to the low boiling points of the proposed solvents, thereby limiting their range of application. Furthermore, the electrolytic materials used in these devices do not allow the electrodeposition of an absorbing and/or reflecting metal layer onto a large area surface without using complicated structures for the cell, such as those described in the articles from S. Zaromb (J. Electrochem. Soc., vol. 109 (1962) pages 903 and 912). The use of some devices is limited to the reflection mode owing to an embodiment where the anode is made of thick metal foils for stability and response speed reasons, and/or to the use of an opaque solid electrolyte. In the case of the devices described in U.S. Pat. No. 3,153,113, a homogeneous absorbing metallic layer is obtained on a surface having an area of one square foot by using a grid as the anode and a superimposed alternative voltage in addition to the DC operating voltage.